Some fungi grow as molds, while others exist in two forms, or dimorphic—either as molds or budding yeast. Fungi can cause pneumonia in healthy people who travel to endemic areas in the United States or Central America; they can also cause rapidly fatal pneumonia in immunocompromised patients recovering from chemotherapy, or HIV-infected patients with advanced disease.
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According to "Harrison's Principles of Internal Medicine," Aspergillus fumigatus commonly invades lung tissue of acute leukemia and bone marrow transplant patients. Ninety percent of immunocompromised patients who develop invasive aspergillosis have two of the following three parameters: granulocyte counts < 500/microliter, treatment with high dose steroids and treatment with cyclosporine. Infection in immunocompromised patients occurs from a break in the skin or through the upper airways. Exposure to environmental sources of Aspergillus commonly occurs at construction sites.
Pneumocystis carinii, renamed P. jiroveci, is an opportunistic fungus that causes pneumonia in HIV-infected patients, or other immunocompromised patients. Pneumocystis has a worldwide distribution.
The fungus Histoplasma capsulatum commonly occurs in the Ohio and Mississippi River Valleys. Outbreaks of histoplasmosis have occurred in college students on spring break in Mexico and in cave explorers in Costa Rica, according to the "American College of Physicians Medical Knowledge Self-Assessment Program." Infection occurs by inhalation of the fungus.
Blastomyces dermatitidis, an endemic fungus, occurs in the southeastern, central and mid-Atlantic areas of the United States, according to "Harrison's Principles of Internal Medicine." Infection occurs by inhalation of the fungus from soil, or rotting wood.
The endemic fungus Coccidioides immitis commonly occurs in California, Arizona and western Texas, according to "Harrison's Principles of Internal Medicine." Outbreaks of coccidioidomycosis have occurred in people building churches in Central America, or participating in archeological digs in Utah, according to the "American College of Physicians Medical Knowledge Self-Assessment Program." Infection in humans occurs by inhalation of the fungus from soil.